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Dep. of Plant and Soil Sciences, 12F Stockbridge Hall, Univ. of Massachussets, Amherst, MA 01003
Dep. of Floriculture and Ornamental Horticulture, Cornell Univ.
Dep. of Ecology and Systematics, Cornell Univ., Ithaca, NY 14853
* Corresponding author.
Carbon isotope discrimination (
) is negatively correlated with water use efficiency (WUE) in several C3 grass species and was proposed as a selection criterion for improving WUE. Our objectives were to determine the relationship between and WUE in cultivars of Kentucky bluegrass (Poa pratensis L., KBG) maintained as turf, and to determine the relationship between cultivar and evapotranspiration (ET) rate measured under non-limiting soil moisture conditions. We view as a potential index of ET and an alternative to weighing lysimeters in selecting C3 turfgrass for reduced ET. WUE of 11 cultivars was evaluated in 25-cm-diam., fritted clay filled lysimeters, over two 4-d harvest periods. Clipping dry weight accumulated above a 50-inm base mowing height for each 4-d period was used as the productivity component in calculating WUE. Gravimetric water loss determined by the water balanced method was used as the transpirational component. Cultivars differed significantly (P 
0.05) in WUE, , and ET, depending on the harvest period. Also, the reliability of in predicting WUE and ET was dependent on the harvest period. During the first harvest period, was inversely related with WUE (r = –0.55**, significant at P 0.01) and cultivar ET varied significantly with (r = 0.64*, significant at P 0.05). During the second harvest period, no significant relationship (P > 0.05) developed between and WUE (r = –0.21), or between and cultivar ET (r = 0.32). Observations for the first and second harvest periods varied considerably in canopy temperature, with temperature ranging from 20.5 to 22.0 ± 1.3°C, and 27.3 to 36.0 ± 3.4°C, respectively. Theory predicts under the assumption of equal leaf temperature, and WUE are correlated. It was likely that leaf temperature differences during the second harvest contributed significantly to the breakdown of the relationship between and WUE. This assumption was likely met during the first harvest period only, therefore and WUE (and ET) were related. Selecting for low appears to have potential for identifying cool season turfgrass for reduced ET. However, further research is needed to determine the reliability of in predicting ET under non-limiting and water limiting conditions.
Received for publication November 3, 1995.
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